Many different types of security documents are utilized in commerce, such as watermarked paper, visible and invisible fibers, optical variable inks, holograms, latent images (e.g. see U.S. Pat. Nos. 4,715,623, 4,033,059 and 4,210,345), security threads with microprinting, and the like, in order to thwart counterfeiters. Latent images have been used in security printing for many years by bank note printers using an intaglio process (utilizing engraved steel printing plates). The latent image was visible when a document printed thereby was viewed at an angle, and the image was caused by the formation of a slight shadow from the raised ink pattern deposited by the engraved printing plate. The shadow was caused by each of the raised ridges that occurred during the printing of the pattern. Most of the printing is done using a 120 line line screen producing a shadow in the direction away from the light source for each of the 120 lines. That is, the shadows so produced on the document create what appears to be a hidden image.
The invention relates to a particular method of providing a latent image, and the document produced thereby, that is relatively easy to practice using conventional materials, is well camouflaged when normally viewed, yet readily revealed to the naked human eye simply by tilting the document (e.g. at an angle of about 45-90 degrees to perpendicular). The invention also is readily adaptable to the inclusion of fluorescent material or brighteners in the ink providing the latent image, so that the latent image may also be revealed by shining a long wave length black light thereon. The latent image produced according to the invention is produced by a combination of line images similar to the intaglio process, but highlights the image when held at an angle to the light by a change in the reflected light. This creates the illusion of different densities of color, revealing the latent image.
According to one aspect of the present invention, a method is provided of producing a security feature on a document disposable in a plane. The method comprises the steps of: (a) making first and second halftone line screens of the same design; (b) superimposing the first and second screens one upon the other so that the designs provided thereon are juxtaposed; (c) producing a copy film having a full value halftone using the screens; (d) making image masks; (e) juxtaposing the image masks and copy film to produce a printing plate; (f) printing the design with latent image on a document using the printing plate; and (g) overprinting at least part of the design with an ink or varnish; and wherein steps (a)-(g) are practiced so as to produce an image in the design on the document which is not readily visible to the naked human eye when viewed generally perpendicular to the plane of the document, but which is readily visible by the naked human eye when the document is tilted so that it is viewed distinctly non-perpendicular to the plane of the document.
Steps (d) and (e) are preferably practiced to make first and second masks, one with an image, and one without, and positioning the masks so that they align with the design on the copy film, and then transferring the design and image from the copy film and masks to the printing plate.
Step (c) is preferably practiced by exposing the film once, then removing the film from registration and rotating the screens at a predetermined angle (e.g. 90, 60, or 45 degrees) and then re-registering the film with the screens and making a second exposure before developing the film. Additional exposures can also be made before developing as necessary to produce a full value halftone.
Step (a) is preferably practiced to produce screens with a frequency of lines in the range of 100-133 lines/inch, e.g. 120 lines/inch; and step (f) by conventional lithographic printing. Step (g) may be practiced by using an opaque white ink or overprint varnish, or a clear, dull, or matte finish ink or varnish. The design produced by the method may be a seal, e.g. having a generally oval configuration, or a border. The image produced may be in the form of an arabic numeral, a letter, or other symbol (e.g. a logo).
Step (g) also may be practiced with a white ink containing a fluorescent material or brightener. The fluorescent material or brightener makes the latent image more visible when viewed at an angle, however then the method may also comprise the further step of shining long wave length black light on the document to reveal the image.
The invention also comprises a document made by the method described above.
According to another object of the present invention, a security paper document having a planar surface is provided. The document comprises: a full value halftone image printed on the planar surface; a clear, dull or matte finish ink or varnish overprinted on at least that portion of the design having the image; and the image not readily visible by the naked human eye when viewed generally perpendicular to said planar surface of the document, but being readily visible by the naked human eye when the document is tilted so that it is viewed distinctly non-perpendicular to the planar surface of the document.
The full value halftone seal may be formed by conventional lithographic printing processes using halftone line screens having a frequency within the range of 100-133 lines/inch. The image ink or varnish may be opaque white ink or overprint varnish, e.g. white ink with a fluorescent material or brightener. The design may be a seal, e.g. one generally oval in shape, and the image may be an arabic numeral, a letter, or other symbol (e.g. logo).
It is the primary object of the present invention to provide a security document with a readily revealed latent image, and a simple yet effective manner of production thereof. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.